Heretofore, pilot operated relief valves have been utilized on pressure vessels, such as tanks, such as shown in U.S. Pat. No. 4,705,065 dated Nov. 10, 1987, for example, in the opening and closing of a main relief valve member and including the opening of the main valve member under a vacuum condition. However, the main relief valve has not functioned independently of the pilot valve such as may be necessary for a severe service and corrosion environment. The main valve member is in a fail open position in the event of any malfunctioning of the pilot valve.
Present pilot operated relief valves can be operated at pressure very near the set pressure without leakage, and general seat tightness under all conditions is superior because of the high seating loads. Weighted and spring loaded relief valve members, however, are very simple and provide consistent reliability for users. Pilot valves which also provide excellent reliability, except under extreme service conditions, are more complex in operation but have superior performance.
Pilot operated relief valves are particularly useful for low pressure vessels designed for a pressure under 15 psi. Such low pressure vessels are required to have: (1) a normal vent (usually small in orifice size) that limits the pressure rise due to either the addition of liquid to the vessel or the increase in vessel vapor space operating ambient temperature, (2) a normal breather (usually small in orifice size) that limits the vacuum on the vessel caused by removal of liquid from the vessel or a reduction in vessel vapor space operating ambient temperature, and (3) an emergency pressure vent (usually large in orifice size) that relieves the pressure in a vessel in the event the tank is subjected to fire or the like. Present code requirements for a low pressure vessel do not permit the setting of a valve above the maximum allowable working pressure (MAWP).Full relief capacity for the emergency vent may be at a pressure of 20% above the MAWP.
Heretofore, pilot operated relief valves have been utilized with the normal vent of a pressure vessel and upon opening of the main valve member under control of the pilot, fluid vapors have normally been vented to atmosphere. Recent requirements of the Environmental Protection Agency (EPA) limit the leakage of harmful vapors from pressure vessels to a maximum concentration of 500 parts per million and these requirements have to be accomplished by Dec. 31, 1992. Thus, it is necessary under certain conditions to collect the leakage of any harmful vapors from the pilot valve, particularly where the pilot valve is set to operate at a set pressure generally around 100% of the MAWP, for example, and this causes a relatively high frequency of operation of the pilot valve resulting in an opening of the main valve member for vapor leakage.